This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce spatially compounded images of moving tissue and/or flow information.
Pending U.S. patent applications Ser. No. 09/335,058, 09/335,159 and 09/335,160 describe apparatus and methods for performing real time spatial compounding of ultrasonic diagnostic image information. Spatial compounding is an imaging technique in which a number of ultrasound images of a given target that have been obtained from multiple vantage points or angles (look directions) are combined into a single compounded image by combining the data received from each point in the compound image target which has been received from each angle. Examples of spatial compounding may be found in U.S. Pat. Nos. 4,649,927; 4,319,489; and 4,159,462. Real time spatial compound imaging is performed by rapidly acquiring a series of partially overlapping component image frames from substantially independent spatial directions, utilizing an array transducer to implement electronic beam steering and/or electronic translation of the component frames. The component frames are combined into a compound image by summation, averaging, peak detection, or other combinational means. The acquisition sequence and formation of compound images are repeated continuously at a rate limited by the acquisition frame rate, that is, the time required to acquire the full complement of scanlines over the selected width and depth of imaging.
The compounded image typically exhibits reduced speckle artifacts and improved delineation of specular reflector interfaces as compared to conventional ultrasound images from a single viewpoint. While these advantages may be readily obtained in the compounding of B mode images, it would be desirable to obtain the same advantages for Doppler imaging. Moreover, the use of different look directions for the component images which are compounded offers the prospect of overcoming a limiting characteristic of Doppler imaging. Doppler signals characteristically have a strong angle dependence. Flow or motion parallel to the transmit beam returns a large Doppler shift, whereas flow or motion orthogonal to the transmit beam provide no Doppler shift. Due to the use of a wall filter the strength of the Doppler signal varies between these extremes. In power Doppler imaging, for instance, where it is desired to image the intensity of the Doppler signal, the clinician cannot be certain that he is observing variation in motion or flow or simply a manifestation of this angle dependence of the Doppler signal. It is desirable to reduce this uncertainty so that more sensitive power Doppler images and more accurate velocity Doppler images can be provided, in addition to providing the aforementioned benefits of compound imaging.
In accordance with the principles of the present invention, Doppler signals are acquired from a number of different look directions and are processed to form real time spatially compounded ultrasonic Doppler images. The Doppler compound imaging technique can be applied to power Doppler or velocity Doppler (colorflow) imaging of either bloodflow or tissue motion. In an illustrated embodiment Doppler image frames are compounded by temporal filtering in estimate space, then converted to the desired display format in display space. The spatially compounded Doppler image may be acquired in an interleaved manner, and combined for display with a spatially compounded or non-compounded B mode image.